Spectrally broadening ultrashort-pulse compressor

Abstract

An ultrashort-pulse compressor includes (a) one or more bulk-optics intersecting a propagation path of an ultrashort-pulsed laser beam multiple times to spectrally broaden a pulse of the laser beam during each of multiple passes through the bulk-optic(s), (b) one or more dispersive optics for compressing a duration of the pulse after each of the multiple passes, and (c) a plurality of focusing elements for focusing the laser beam between the multiple passes. Propagation distances between the bulk-optic(s) and the focusing elements are detuned from imaging such that a spot size of the laser beam, at the bulk-optic(s), is greater at each successive one of the multiple passes. As the laser beam propagates through this compressor, each laser pulse is alternatingly spectral broadened and temporally compressed. The increasing spot size of the laser, for each pass, helps prevent optical damage, run-away self-focusing, and other undesirable outcomes.

What is claimed is: 
     
       1. An ultrashort-pulse compressor, comprising:
 one or more broadening bulk-optics arranged to intersect a propagation path of an ultrashort-pulsed laser beam multiple times so as to spectrally broaden a pulse of the ultrashort-pulsed laser beam during each of multiple passes through the one or more broadening bulk-optics; 
 one or more dispersive optics for compressing a duration of the pulse after each of the multiple passes; and 
 a plurality of focusing optics for focusing the ultrashort-pulsed laser beam between the multiple passes, the plurality of focusing optics being arranged around the one or more broadening bulk-optics; 
 wherein distances, along the propagation path, between successive passes through the one or more broadening bulk-optics are detuned from imaging such that a spot size of the ultrashort-pulsed laser beam at the one or more broadening bulk-optics is greater at each successive one of the multiple passes. 
 
     
     
       2. The compressor of  claim 1 , wherein, for each pair of successive passes:
 the propagation path between the successive passes goes from one pass through the one or more broadening bulk-optics via one of the plurality of focusing optics, having focal length f, to one of the dispersive optics and then via one of the plurality of focusing optics, also having focal length f, to another pass through the one or more broadening bulk-optics; and 
 length of the propagation path between the successive passes is 2(2f+x), absolute value of x being greater than zero and less than f. 
 
     
     
       3. The compressor of  claim 2 , wherein, said one of the dispersive optics is at a midpoint of the propagation path between the successive passes. 
     
     
       4. The compressor of  claim 2 , wherein the absolute value of x is less than 0.1f. 
     
     
       5. The compressor of  claim 1 , wherein:
 the dispersive optics include a plurality of chirped mirrors each positioned on the propagation path between a respective pair of successive passes; 
 the plurality of focusing optics include a plurality of concave mirrors each positioned on the propagation path between a respective pair of successive passes; and 
 each of the multiple passes goes through a same one or same ones of the one or more broadening bulk-optics. 
 
     
     
       6. The compressor of  claim 5 , wherein the one or more broadening bulk-optics consist of a single broadening bulk-optic. 
     
     
       7. The compressor of  claim 5 , wherein the one or more broadening bulk-optics include a pair of wedge-shaped broadening bulk-optics having a combined thickness in the ultrashort-pulsed laser beam that is adjustable by changing positions of the wedge-shaped broadening bulk-optics relative to each other. 
     
     
       8. The compressor of  claim 5 , wherein:
 the concave mirrors include first and second concave mirrors facing opposite first and second sides, respectively, of said same one or same ones of the one or more broadening bulk-optics; and 
 the chirped mirrors include first and second chirped mirrors respectively facing the first and second concave mirrors such that the propagation path makes several repetitions between the first and second chirped mirrors via the first and second concave mirrors and through said same one or same ones of the one or more broadening bulk-optics. 
 
     
     
       9. The compressor of  claim 8 , wherein length of the propagation path, between said same one or same ones of the one or more broadening bulk-optics, and each of the first and second chirped mirrors, exceeds two times a focal length of a corresponding one of the first and second concave mirrors by a fraction of the focal length. 
     
     
       10. The compressor of  claim 8 , wherein:
 length of the propagation path, between the first concave mirror and the first chirped mirror, equals a focal length of the first concave mirror; 
 length of the propagation path, between the first concave mirror and said same one or same ones of the one or more broadening bulk-optics, deviates from the focal length of the first concave mirror by a fraction of the focal length of the first concave mirror; 
 length of the propagation path, between the second concave mirror and the second chirped mirror, equals a focal length of the second concave mirror; and 
 length of the propagation path, between the second concave mirror and said same one or same ones of the one or more broadening bulk-optics, deviates from the focal length of the second concave mirror by the same or another fraction of the focal length of the second concave mirror. 
 
     
     
       11. The compressor of  claim 8 , further including:
 an initial focusing optic for focusing the ultrashort-pulsed laser beam prior to a first one of the multiple passes; and 
 a final focusing optic for collimating the ultrashort-pulsed laser beam after a last one of the multiple passes. 
 
     
     
       12. The compressor of  claim 11 , further comprising:
 a first steering mirror arranged to direct the ultrashort-pulsed laser beam to said same one or same ones of the one or more broadening bulk-optics, after being focused by the initial focusing optic; and 
 a second steering mirror arranged to direct the ultrashort-pulsed laser beam to the final focusing optic after the last one of the multiple passes. 
 
     
     
       13. The compressor of  claim 8 , wherein the propagation path reaches one of the first and second chirped mirrors after a last one of the multiple passes. 
     
     
       14. The compressor of  claim 8 , wherein:
 the propagation path reaches neither the first nor the second chirped mirror after a last one of the multiple passes; and 
 the dispersive optics further include a third dispersive optic arranged to compress the pulse after the last one of the multiple passes. 
 
     
     
       15. The compressor of  claim 1 , wherein each of the multiple passes has a B-integral that is within 30% of the B-integral averaged over the multiple passes. 
     
     
       16. The compressor of  claim 1 , wherein the one or more broadening bulk-optics include a plurality of broadening bulk-optics alternating with the dispersive optics along the propagation path. 
     
     
       17. The compressor of  claim 1 , wherein the one or more dispersive optics and the plurality of focusing optics are arranged such that the propagation path of the ultrashort-pulsed laser beam, between each pair of successive passes through the one or more broadening bulk-optics, goes first to one of the plurality of focusing optics, then to one of the one or more dispersive optics, and then to one of the plurality of focusing optics. 
     
     
       18. The compressor of  claim 1 , wherein the one or more dispersive optics are separate from the one or more broadening-bulk optics, and the plurality of focusing optics are separate from both the one or more broadening-bulk optics and the one or more dispersive optics. 
     
     
       19. A method for compressing an ultrashort pulse of a laser beam, comprising:
 repeating a group of steps of:
 spectrally broadening and chirping the ultrashort pulse in one or more broadening bulk-optics, 
 dechirping the ultrashort pulse, after the spectrally broadening step, to compress duration of the ultrashort pulse, and 
 focusing the laser beam to set a spot size of the laser beam on the one or more broadening bulk-optics in the spectrally broadening step; 
 wherein propagation distances of the laser beam between the spectrally broadening step of each successive repetition of the group of steps are detuned from imaging such that the spot size, in the one or more broadening bulk-optics, is greater for each successive repetition of the group of steps. 
 
 
     
     
       20. The method of  claim 19 , wherein:
 in each but a first repetition of the group of steps, the focusing step comprises two steps of modifying divergence angle of the laser beam with a focusing optic having a focal length f, such that the laser beam forms a waist between the two steps of modifying divergence angle; and 
 a propagation path of the laser beam between each pair of successive repetitions of the spectrally broadening step has length 2(2f+x) with x having absolute value greater than zero and less than f. 
 
     
     
       21. The method of  claim 20 , wherein the absolute value of x is less than 0.1f. 
     
     
       22. The method of  claim 20 , wherein the propagation path has length f+x from the first one of the pair of successive repetitions of the spectrally broadening step to the first one of the two steps of modifying divergence angle, length 2f between the two steps of modifying divergence angle to the compressing step, and length f+x from the second one of the two steps of modifying divergence angle to the second one of the pair of successive repetitions of the spectrally broadening step. 
     
     
       23. The method of  claim 22 , wherein, within each repetition of the group of steps, a same focusing optic performs each of the two steps of modifying divergence angle steps, and the compressing step is performed between the two steps of modifying divergence angle. 
     
     
       24. The method of  claim 19 , wherein a same one or same ones of the one or more broadening bulk-optics performs each successive repetition of the spectrally broadening step. 
     
     
       25. The method of  claim 24 , wherein each successive repetition of the spectrally broadening step imparts, on the laser beam, a B-integral that is within 30% of the B-integral averaged over the successive repetitions. 
     
     
       26. The method of  claim 24 , wherein each successive repetition of the spectrally broadening step imparts, on the laser beam, a B-integral that is less than 2.0 radians. 
     
     
       27. The method of  claim 24 , wherein the compressing step is performed by one or more chirped mirrors in at least each successive repetition that is not a last repetition. 
     
     
       28. The method of  claim 27 , wherein the focusing step is performed by a concave mirror in each successive repetition that is not a first repetition. 
     
     
       29. The method of  claim 24 , wherein the one or more broadening bulk-optics include a pair of wedge-shaped broadening bulk-optics, and the method further comprises a step of adjusting a combined thickness of the pair of wedge-shaped broadening bulk-optics in the laser beam by changing positions of the wedge-shaped broadening bulk-optics relative to each other. 
     
     
       30. The method of  claim 19 , wherein the step of dechirping is performed with one or more dispersive optics separate from the one or more broadening bulk-optics, and the step of focusing is performed with a plurality of focusing optics separate from both the one or more broadening bulk-optics and the one or more dispersive optics.